Road curvature measurement is typically performed by specialized surveying systems. Such systems combine a local curvature measurement (coming from a yaw rate sensor or vehicle wheel speed sensors) with global positioning satellite (GPS) location signals, and assign curvature to various positions as the vehicle passes over an area of interest. There is no look-ahead functionality associated with such systems—the curvature measured is that at the vehicle position.
Classical image processing based approaches to curvature measurement use the Hough transform, least-squares parabolic fitting, or least-squares circle fitting to determine the curvature, passing a polynomial through three or more points that are deemed to define the road. However, such methods require at least three measurement points to function, which are not always available.
U.S. Pat. No. 5,675,489 provides a method that receives an image of the road ahead and uses a search procedure, straightening out the image to a perspective -free (map-like) image of road lane markings, to determine the road curvature. Not only is a search procedure required for this, but the method relies on computationally intensive interpolation between pixel gray levels for fractional shifting.